A number of different dust lifting parameterizations have been used to model the injection of dust from the Martian surface into the atmosphere, and the form of the resulting dust cycles and dust storms produced are found to be highly dependent on the precise form of the parameterization used, provided that it includes some threshold dependence, and particularly where radiatively active dust transport is employed. This talk will review the most interesting results from previous work. We have recently altered a key factor which particularly affects the dust lifting due to near-surface wind stress, however, so we will also present results using the new dust lifting formulation, and make some comparisons

Forget, F.

Lewis, S.R.

Newman, C.E.

Read, P.L.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDust cycles and storms in a Mars GCMConference or Workshop ItemHow to cite:Newman, C.E.; Read, P.L.; Lewis, S.R. and Forget, F. (2003). Dust cycles and storms in a Mars GCM. In:First International Workshop on Mars atmosphere modelling and observations, 13-15 Jan 2003, Granada, Spain.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www-mars.lmd.jussieu.fr/granada2003/abstract/newman dust.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDUST CYCLES AND STORMS IN A MARS GCM.C. E. Newman, P. L. Read, S. R. Lewis, Atmospheric, Oceanic and Planetary Physics, Department of Physics,University of Oxford, England (newmanc@atm.ox.ac.uk), F. Forget, Laboratoire de Me´te´orologie Dynamique duCentre National de la Recherche Scientifique, Universite´ Paris 6, 4 place Jussieu, 75252 Paris Cedex 05, France..IntroductionA number of different dust lifting parameterizations havebeen used to model the injection of dust from the Martiansurface into the atmosphere, and the form of the result-ing dust cycles and dust storms produced are found tobe highly dependent on the precise form of the parame-terization used, provided that it includes some thresholddependence, and particularly where radiatively activedust transport is employed. This talk will review themost interesting results from previous work. We haverecently altered a key factor which particularly affectsthe dust lifting due to near-surface wind stress, however,so we will also present results using the new dust liftingformulation, and make some comparisons.In the boundary layer, the drag velocity umay begiven by (see e.g. [1])udrag=ku(z)ln zz0: (1)wherek=von Karman’s constant=0.4,z=height abovethe surface, z0=roughness height=0.01m and u(z) is themagnitude of the near-surface wind velocity (at heightz). In the near-surface wind stress lifting parameteriza-tion, surface particles are assumed to be lifted by near-surface wind stress whenever u=p(stress=density)exceeds a threshold value ut, which may be calculatedusing a set of semi-empirical equations [2]. For ex-pected values of interparticle cohesion, the threshold ispredicted to be lowest, thus allowing the greatest move-ment, for large 100m particles. We therefore calcu-late utfor these particles, and assume that the smallerdust particles are lifted via saltation. We then assumethat the vertical dust flux F is proportional to the hori-zontal flux of saltating particles [3], givingF = N 2:61g(u)31 utu1 +utu2: (2)A simpler (though less accurate) alternative methodis to find utusing a constant threshold stress, and thenuse equation 2 as before. This has the advantage ofhaving only one free parameter (the constant thresholdstress used) which enables parameter space to be ex-plored more rapidly.Lifting by the convective vortices known as dustdevils is accounted for in two ways, both of which in-volve modelling the vortices as convective heat engines[4]. In the first method, dust lifting is set proportionalto the ‘dust devil activity’, which is related mostly tothe thickness of the convective boundary layer and thesurface sensible heat flux, and no thresholds are used.This results in a relatively smooth variation of dust devillifting over the planet, and represents a background lift-ing process. Computationally, it may also prevent themodel failing due to unrealistically strong gradients intemperature, etc. – these can be produced by overly lo-calised dust distributions, which sometimes arise whennear-surface wind stress lifting is confined to one or twogridpoints. The second method uses the heat enginemodel to predict the tangential velocity V around a vor-tex, again depending on the thickness of the convectiveboundary layer and the surface sensible heat flux, butnow dust is only lifted if this exceeds a threshold value,Vt, calculated using a semi-empirical equation derivedfrom laboratory dust devil experiments [2].Results using the previous formulationIn the previous formulation, uvalues were calculatedfrom values of u(z) found prior to vertical diffusionbeing carried out on the model wind field. The mainresults using this formulation were as follows:If dust is lifted only by near-surface wind stress, orwith the first dust devil lifting parameterization used inaddition to produce reasonable background dust load-ing, certain ‘typical’ Martian dust storms are producedin some experiments. The most interesting behaviouris found when a high threshold utis used (this is thecase if the interparticle cohesion between dust particlesis assumed to be large) as this allows for dust liftingto occur suddenly and in localised regions when thesethresholds are exceeded. Regional storms are often sim-ulated to begin in the Hellas region, where the rapidgrowth produced is due largely to the positive feedbackson surface wind speeds at the edge of the growing dustcloud, where there are large temperature contrasts. Fig-ures 1 and 2 show respectively visible dust opacities andzonally averaged mixing ratios for one such simulation.Figure 3 shows visible dust opacities for the growthphase of another type of storm, beginning near theChryse region, in which the dust cloud grows as it movessouthwards in a strong western boundary current to theeast of Tharsis. Again there is a positive feedback be-tween the winds at the cloud’s edge and dust lifting,although the latter finishes once the storm reaches thesouthern hemisphere, and it decays over the south pole.These two regional storms are similar to those ob-MARS GCM DUST CYCLES: Newman et al.Figure 1: Visible dust opacities during a simulated storm be-ginning in the Hellas region at Ls185 degrees. Also shownis the 500Pa surface pressure contour, to indicate roughly theposition of topography.served on Mars [5], [6], yet other types of storms, thosebeginning in other locations, or those which go on tobecome global in extent, do not appear to be easily pro-duced using the near-surface wind stress lifting param-eterization alone, at least with this formulation. TheChryse storm type also represents the greatest source ofinterannual variability produced in these simulations, asit does not occur in every year, yet on real Mars there isfar more variability observed.If dust is lifted by a combination of near-surfacewind stress and the second dust devil lifting parameteri-zation, other interesting behaviour is seen. Near-surfacewind stress lifting tends to produce a positive feedbackbetween increased dustiness and further lifting (due tolocal wind speed increases, or increases in the strengthof the global circulation), but dust devil lifting tendsto have a negative feedback (as increased dustiness re-duces the surface heating, hence reduces the drive to theFigure 2: Zonally averaged dust mixing ratios correspondingto the dust opacities shown in figure 1.dust devils). The interplay between the two processestherefore produces increased variability on timescalesof a few sols, and increases the potential for interannualvariability. Dust devil lifting also peaks in different re-gions to near-surface wind stress lifting in many cases,so increases the number of main storm onset regions.The peak in dust lifting also shifts from region to region,as is often observed on Mars, and the areas involvedare those commonly observed to be dusty during largestorms [7].With near-surface wind stress lifting only, stormswere produced which grew realistically rapidly and be-came global, but in general they then went on to attainunrealistically high opacities without decaying. Theadvantage of combining this with the second (threshold-sensitive) form of dust devil lifting is that the negativefeedbacks involved for the dust devil mechanism mod-erate this growth, and thus global storms which decayspontaneously are easily produced. Figure 4 shows zon-ally averaged visible dust opacities averaged over lati-MARS GCM DUST CYCLES: Newman et al.Figure 3: Visible dust opacities during a simulated storm be-ginning in the northern Chryse region at Ls240 degrees.tude bands for three years of the experiment describedabove. This simulation is still deficient, however, in thatit does not produce any ‘clear’ years, only years with oneor more global storms, and northern spring and summeropacities in particular are far higher than observed. Thelatter is due mostly to dust devil lifting,and seems to sug-gest that dust devils cannot be responsible for as high aproportion of the dust lifting as this simulation allows,unless some other mechanisms (e.g. scavenging by iceclouds) removes much of the dust lifted at this time.Results using the newer formulationIn the newer formulation, uvalues are calculated fromvalues of u(z) found after vertical diffusion is carriedout on the model wind field. Initial results indicate thatusing uafter vertical diffusion has been applied reduceswind stresses considerably, thus requiring lower thresh-olds than before to produce comparable opacities. Usingpost-vertical diffusion ualso, however, removes suspi-ciously high northern hemisphere lifting peaks (partic-ularly in regions of large topographic gradients) whichwere primarily responsible for global storms growinguncontrollably in the case of near-surface wind stress180 270 0 90 180 270 0 90 180 270 0 90 180Areocentric longitude Ls012345Visible optical depth at 610 Pa averaged over latitude band67.5 N - 90 N45 N - 67.5 N22.5 N - 45 N0 - 22.5 N22.5 S - 045 S - 22.5 S67.5 S - 45 S90 S - 67.5 SFigure 4: Zonally averaged visible dust opacities in latitudebands for three years of a simulation with threshold-sensitivenear-surface wind stress and dust devil lifting. Two globalstorms are produced during the first year which compare wellwith the 1977a and b storms observed by Viking.lifting experiments. Also, the Hellas and Chryse duststorms shown above are still found to occur, but addi-tional regions (such as Noachis, where a regional stormwas observed in 1997, see [8]) are found to be peak dustsources at different times of year. This work is still inthe early stages, but new results will be presented at theworkshop.DiscussionThe previous formulation produced some reasonably re-alistic dust storms and dust opacity cycles, but thesewere still deficient in some respects, and problems wereencountered with excessive near-surface dust lifting viapositive feedbacks, particularly in the northern hemi-sphere and at higher model resolutions. The newer for-mulation may produce a greater variety of stable duststorms via the near-surface wind stress parameteriza-tion (thought to be the major contributor to dust liftingon Mars), including global storms which do not growuncontrollably.MARS GCM DUST CYCLES: Newman et al.References[1] Garratt, J. R., The Atmospheric Boundary Layer,Cambridge University Press, 1994.[2] Greeley, R. and J. D. Iversen, Wind as a GeologicalProcess on Earth, Mars, Venus, and Titan, CambridgeUniversity Press, 1985.[3] White, B. R., Soil Transport By Winds On Mars,J.Geophys.Res., vol.84, no.B9, pp.4643-4651, 1979.[4] Renno´, N. O., M. L. Burkett and M. P. Larkin, A sim-ple thermodynamical theory for Dust Devils, J.Atmos.Sci.,vol.55, pp.3244-3252, 1998.[5] Capen, C. F., Martian yellow clouds – past and future,Sky & Telescope, vol.41, pp.117-120, 1971.[6] Smith, M. D., J. C. Pearl, B. J. Conrath, P. R.Christensen, Thermal Emission Spectrometer results:Atmospheric thermal structure and aerosol distribution,J.Geophys.Res., vol.106, no.E10, pp.23929-23946,2001.[7] Thorpe, T. E., A history of Mars atmospheric opacityin the southern hemisphere during the Viking extendedmission, J.Geophys.Res., vol.84, no.A11, pp.6663-6683,1979.[8] Smith, M. D., J. C. Pearl, B. J. Conrath and P. R.Christensen, Mars Global Surveyor Thermal EmissionSpectrometer (TES) observations of dust opacity dur-ing aerobraking and science phasing, J.Geophys.Res.,vol.104, no.E4, pp.9539-9552, 2000.

Dust cycles and storms in a Mars GCM

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Dust cycles and storms in a Mars GCM

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